add a root option 'icon' for volume root icon (value is icon URL)

ex.

```php
<?php

$opts = array(
    'locale' => '',
    'roots'  => array(
        array(
            'driver' => 'LocalFileSystem',
            'path'   => '/path/to/files/',
            'URL'    => 'http://localhost/to/files/',
            'icon'    => 'http://localhost/to/img/volume_icon.png' // 16x16 icon
        )
    )
);

// run elFinder
$connector = new elFinderConnector(new elFinder($opts));
$connector->run();
```